I've got an iPhone app that's mainly targetting 3.0, but which takes advantage of newer APIs when they're available. Code goes something like this:

if (UIApplicationDidEnterBackgroundNotification != NULL)
    [nc addObserver: self selector: @selector(irrelelvantCallbackName:) name: UIApplicationDidEnterBackgroundNotification object: nil];

Now, according to everything Apple's ever said, if the relevant APIs are weakly linked, that will work fine because the dynamic linker will evaluate UIApplicationDidEnterBackgroundNotification to NULL. Except that it doesn't. The application compiles, but as soon as it hits "if (UIApplicationDidEnterBackgroundNotification != NULL)" it crashes with EXC_BAD_ACCESS.

Is this simply a matter of a compiler flag I need to set? Or am I going about this the wrong way?